One-time programmable (OTP) memories are used in integrated circuits for a variety of applications including nonvolatile memory applications. They may be used as a single memory cell, or in arrays of memory cells to provide unique chip identifications and to set operating parameters such as clock multipliers and voltage levels for devices such as microcontrollers and microprocessors, and also high-density memory applications. They may also be used to configure, customize, and repair a chip after testing, in order to repair a controller's cache memory array. One-time programmable memories are typically implemented using charge storage, fuse, or anti-fuse approaches. Charge storage approaches have typically involved defining a bit value based on charge stored on an insulated metal-oxide semiconductor (MOS) type gate structure. Such charge storage approaches, however, are not practicable with current and deep sub-micron technologies that feature very thin gate oxide because of the high gate leakage current that prevents a long retention time of the information. And other type of OTP memory is the capacitor type, wherein oxide breakdown is used to program the capacitor type memory cells, as published, U.S. Pat. No. 7,102,951, U.S. Pat. No. 6,421,293 and U.S. Pat. No. 7,046,569.
In FIG. 1A, the capacitor type OTP memory cell is illustrated, as a prior art, wherein a select line (word line) 111 is connected to the gate of MOS transistor 101, a plate line 112 is connected to one plate of a capacitor 102, the other plate 102 of the capacitor is connected to the drain 103 of the MOS transistor, the source 105 of the MOS transistor is connected a data line 113 (bit line). One of major problem of the structure is that the gate oxide of the MOS transistor 101 is damaged during high voltage program. In order to breakdown the oxide of the capacitor 102 during program, the plate line 112 is asserted to high voltage when the MOS transistor 101 is turned on. Thus, the oxide of the capacitor 102 is broken. After broken, the drain 103 is raised near plate line. At the same time, the applied voltage of the gate is also raised. Even though thick oxide is used for the gate, the high voltage may damage the MOS transistor 101 in the deep sub-micron process. In order to avoid the high voltage stress, there was an effort to add one more MOS transistor as published, U.S. Pat. No. 6,927,997. However, this structure can only reduce high voltage stress but it can not remove the high voltage stress, and also increases area.
Still there is a need to improve the access device of the OTP memory cell, and also reduce the process cost. As explained above, the conventional switching (access) devices are based on three-terminal field-effect transistor. In order to stand high voltage stress during program and also to obtain fast switching with low process cost, the MOS access device is replaced with a diode in the present invention. Four-terminal diode can be used as an access device, which includes two bipolar transistors inside. The four-terminal diode is known as Shockley diode or thyristor, is a solid-state semiconductor device similar to two-terminal p-n diode, with an extra terminal which is used to turn it on. Once turned on, diode (p-n-p-n diode or n-p-n-p diode) will remain on conducting state as long as there is a significant current flowing through it. If the current falls to zero, the device switches off. Diode has four layers, with each layer consisting of an alternately p-type or n-type material, for example p-n-p-n and n-p-n-p. The main terminals, labeled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to one of the middle layers. The operation of a diode can be understood in terms of a pair of tightly coupled transistors, arranged to cause the self-latching action.
Diodes are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of polarity of the current causes the device to automatically switch off; referred to as ‘zero cross operation’. The device can also be said to be in synchronous operation as, once the device is open, it conducts in phase with the voltage applied over its anode to cathode junction. This is not to be confused with symmetrical operation, as the output is unidirectional, flowing only from anode to cathode, and so is asymmetrical in nature. These properties are used control the desired load regulation by adjusting the frequency of the trigger signal at the gate. The load regulation possible is broad as semiconductor based devices are capable of switching at extremely high speeds over extremely large numbers of switching cycles.
In FIG. 1B, the schematic of p-n-p-n diode is illustrated. It consists of four terminals, such that the anode 131 is connected to power supply or regulating node, the base 132 of p-n-p transistor 135 serves as the collector 132 of n-p-n transistor 134, the collector 133 of p-n-p transistor 135 serves as the base of n-p-n transistor 134 which is controlled by the voltage controller 136. In order to turn on diode and hold the state of turn-on, the voltage controller should raise the voltage from ground level to VF (forward bias, 0.6 v˜0.8 v for silicon). And the voltage controller 136 should supply the current 137, referred as the base current, which current depends on the characteristic of transistor 134 and 135. Once the base current 137 establishes the forward bias (VF), the collector 132 of n-p-n transistor 134 holds the current path 139 from the base of p-n-p transistor 135. After then, p-n-p transistor 135 is turned on because the base 132 has forward bias from the emitter 131. This sets up the current path 138 which can keep the turn-on state. This is the holding state as long as the base has not so much leakage to drive the base voltage under forward bias (VF) even though the voltage controller 136 is open. To turn off diode, the voltage controller 136 should lower the voltage of the base of n-p-n transistor 134 under forward bias. To do so, the voltage controller 136 should (negatively) flow more current than the current path 138.
In the present invention, four-terminal diode replaces the MOS (Metal-Oxide Semiconductor) field-effect transistor as a switching element. However four-terminal diode can not easily replace the MOS transistor as a switching device because it has unidirectional current control characteristic and internal feedback loop. Now the present invention devotes to replace a MOS transistor with four-terminal diode and sophisticated circuit techniques are introduce to control the diode for the one-time programmable memory. The four-terminal diode can work for the one-time programmable memory as a switching device. It gives as many as advantages to design and fabricate on the wafer.
In the conventional MOS field-effect transistor, there is a parasitic bipolar transistor, as shown in FIG. 1A, wherein the base 104 controls the collector/emitter 103 and 105, and the base 104 serves as a body of the MOS transistor 101. The parasitic bipolar transistor is not wanted device in the conventional MOS transistor which is usually turned off, but now adding one more terminal to the parasitic bipolar transistor, a p-n-p-n diode (or n-p-n-p) can serve as a switching device for the next generation memory.